Why do I have sex dreams about my friends and co-workers?

I wasn't sure whether to put this here or in r/Askreddit, so I put it here. LUCKY YOU.

Anyways, from time to time I will have a graphic sex dream about someone whom I am not even remotely attracted to. The dude who works at Arby's, The guy who sold me a frying pan, My gay friend, the older receptionist lady (!?), They're all fair game. It makes me uncomfortable, and the people I dreamt about don't know why I am being all awkward and weird about it. And when you tell your gay friend that you dreamt about him and his huge wing-wang, he just laughs and stops inviting you to parties.

I know everybody has these dreams sometimes. If you say you don't, you're a big fat liar.